drm/ast: Use drm_atomic_helper_commit_tail() helper

Ast has no special requirements for runtime power management. So
replace drm_atomic_helper_commit_tail_rpm() with the regular helper
drm_atomic_helper_commit_tail().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240627153638.8765-9-tzimmermann@suse.de